The human receptor for interleukin-2 (IL-2, T-cell growth factor) has been molecularly cloned and biochemically characterized. The human genome normally contains a single copy of this receptor gene, however, when transcribed, two mRNAs are produced which vary in length due to use of different polyadenylation signals. Sequence analysis of cloned cDNAs also indicates the presence of alternate pathways of mRNA splicing. Splicing of a 216 base pair intron contained within the coding region of this gene results in an altered protein unable to bind IL-2. In contrast, cDNAs corresponding to unspliced mRNA when ligated to SV-40 regulatory elements and transfected into COS-1 cells result in the expression of membrane receptors capable of binding IL-2 and anti-Tac. The receptor is composed of 272 amino acids including a signal peptide of 21 amino acids. The protein backbone (33,000 daltons) is modified cotranslationally by addition of N-linked carbohydrate then exported to the Golgi and membrane where O-linked sugar, sialic acid, phosphate and sulfate are added. 30,000-60,000 IL-2 receptors are displayed on the surface of PHA activated lymphoblasts while leukemic T cells infected with human T cell leukemia-lymphoma virus express 5-10 fold more receptors. The number of receptors in normal activated T cells, but not HTLV transformed T cells, markedly declines during long term culture suggesting that T cell responsiveness is regulated not only by the availability of IL-2 but also by the expression of IL-2 receptors. Reexpression of receptors occurs following stimulation with antigen or lectin or with agents that activate protein kinase C (phorbol diesters and phospholipase C). The presence of large numbers of IL-2 receptors on the surface of HTLV infected leukemic T cells has been exploited to selectively kill these cells using anti-IL-2 receptor antibody (anti-Tac) coupled to the toxic A chain of ricin.